In recent years, optical coherence tomography (OCT) for forming images that represent surface and/or internal morphologies of objects by using light beams from laser light sources etc. has attracted attention. Unlike X-ray CT, optical coherence tomography is noninvasive to human bodies and is therefore expected to be utilized in medical and biological fields in particular. For example, in ophthalmology, apparatuses for forming images of fundus, cornea, etc. are in practical stages.
An apparatus disclosed in Patent Document 1 uses so-called “Fourier Domain OCT” technique. More specifically, this apparatus irradiates low-coherence light beam to an object, superposes its reflected light and reference light to generate interference light, and acquires spectral intensity distribution of the interference light and executes Fourier transform to image morphology in a depth direction (z-direction) of the object. Further, this apparatus is provided with a galvano mirror for scanning light beams (signal light) along one direction (x-direction) perpendicular to the z-direction, and forms an image of a desired measurement target region of the object. An image formed by this apparatus is a two-dimensional cross-sectional image along the depth direction (z-direction) and scanning direction (x-direction) of the light beam. Such a technique is specifically called Spectral Domain.
Patent Document 2 discloses a technique that scans signal light in horizontal and vertical directions (x-direction and y-direction) to form two-dimensional cross-sectional images along the horizontal direction, and acquires three-dimensional cross-sectional information of a measured area based on these cross-sectional images to perform imaging. Such three-dimensional imaging techniques include, for example, a method that arranges and displays cross-sectional images along the vertical direction (referred to as stack data etc.), method that executing rendering processing on volume data (voxel data) created from stack data to form a three-dimensional image.
Patent Documents 3 and 4 disclose other types of OCT. An OCT apparatus disclosed in Patent Document 3 scans wavelengths of light irradiated to an object (wavelength sweeping), detects interference light obtained by superposing reflected lights of the respective wavelengths on reference light to acquire spectral intensity distribution, and executes Fourier transform on it to image morphology of an object. Such an OCT technique is called Swept Source etc. Swept Source OCT is a kind of Fourier Domain OCT.
An OCT apparatus disclosed in Patent Document 4 irradiates light having predetermined beam diameter to an object and analyzes components of interference light obtained by superposing reflected light thereof and reference light, thereby forming an image of a cross section of the object orthogonal to irradiating direction of the light. Such an OCT technique is called Full-Field, En-face, etc.
Patent Document 5 discloses an example of OCT application to ophthalmology. Before OCT was applied, retinal cameras, slit lamp microscopes, scanning laser ophthalmoscopes (SLO) etc. were used for observing eyes (see Patent Documents 6 to 8 for example). Retinal cameras photograph fundus by irradiating illumination light onto an eye and receiving reflected light from the fundus. Slit lamp microscopes obtain a cross-sectional image of cornea by cutting off a light section of the cornea using slit light. SLO images morphology of retinal surface by scanning fundus with laser light and detecting reflected light by high-sensitive elements such as a photomultiplier. These modalities photograph fundus, cornea, etc. from the front to acquire images (front images).
OCT apparatuses have advantages over retinal cameras etc. in that high-definition images may be obtained, cross-sectional and three-dimensional images may be obtained, etc.
Because OCT apparatuses may be used for observing various sites of eyes and is capable of obtaining high-definition images in this way, they have been applied to diagnoses of various ophthalmologic disorders.